Street photos of homeless persons. Black and white for a dramatic effect. It is only a suggestion though. Anything single; flower (as already suggested) or an older person such as a grandma or grandpa that is now single through death of a spouse. If you want to do lonely you need to understand it so that you can deal with it in a humane way.

If you really want to do lonely try, in a professional and compassionate manner, to shoot a spouse or child of an alsheimer patient.

Just suggestions from me that take a bit more effort and sensitivity. Maybe a lot more depending on how you are equipped to handle theses circumstances.

I like what crotograph said, about needing to understand loneliness to best capture it. And, I have to agree. As someone who lost a spouse, it's really awfully easy for me to grasp loneliness. I wish it wasn't so easy, but that's the way it goes. I definitely feel that I'm more capable of capturing loneliness now than before she died.

Saw this girl a while ago siting on a little jetty thing next to the river while i was down the boardwalk one day - she was sorta just sitting there looking into the water - That reminded me in a way of lonlieness/sorrow at the time

EDIT: Okay, more seriously, I think a big city full of people can be a very lonely place if you don't know anybody in it. Hence I might consider an urban situation with lots of people in it either all looking lonely together or with one person in the image looking lonely whilst all around them get on really well. Probalem with the last one is that it might be difficult not to look too contrived.

Often a head shot of a person looking to one side has them staring onto the empty space. How about the empty space being behind the subject with them looking down to the ground shoulders very slightly tensed. Set it outside on a cold bleak looking day (in a rural landscape) and that might work for you...

A famous musician once said "You don't play the blues, you tell the blues". As with Trenton, it is easier to visualize what you feel. I'm not telling you to go break up or get a divorce...:mrgreen:, but try to walk in the lonely man's shoes, and the visualization and emotion will help with the expression.